Monday was an important date on the way to the 2018 elections. It was the beginning of the period when Illinois candidates have to file petitions with, in some cases, thousands of signatures needed to get on the ballot. The filing period lasts a week, but for hundreds of candidates, it was worth it to be in line first thing Monday morning.

On this encore edition of The 21st: The book 'We Have No Idea' illustrates all that we still don't know about the universe. Plus, we revisit our conversation about the Trail Of Tears' history in southern Illinois, and hear from a Mexican restaurant owner on what it's like to operate his business in this political climate.

Facebook is facing tough questions in the wake of the 2016 presidential election. Did Russians use social media to sway the election? Why is there so much fake news? Why isn’t Facebook more transparent?